The present invention relates to an amplifier, for example for use at radio frequencies as a low noise amplifier (LNA) in a tuner. The present invention also relates to a tuner including such an amplifier.
EP 0 716 505 discloses an LNA comprising first and second transconductance stages and a series feedback arrangement. The first transconductance stage amplifies the radio frequency input signal whereas the second stage is in a negative feedback loop of the first stage. In particular, the input of the second stage is connected to the output of the first stage whereas the output of the second stage is connected to a series feedback resistor of the first stage. This has the effect of increasing feedback around the first stage, and hence reducing distortion, while maintaining a good noise figure.
According to a first aspect of the invention, there is provided an amplifier comprising: an amplifier input; an amplifier output; a first transconductance stage having a first input connected to the amplifier input, a current feedback input, and a first output connected to the amplifier output; a second transconductance stage having a second input connected to the amplifier output and a second output connected to the current feedback input to provide negative feedback; and a third transconductance stage having a third input connected to the amplifier input and a third output connected to the amplifier output to provide feedforward error correction.
The amplifier may comprise a radio frequency amplifier, such as a low noise amplifier.
The amplifier may comprise a load connected to the amplifier output.
Each of the first, second and third transconductance stages may comprise a differential transconductance stage having differential inputs and outputs.
The first transconductance stage may comprise first and second transistors whose input terminals comprise the first input, whose output terminals comprise the first output, and whose common terminals comprise the current feedback input. A first resistance may be connected between the common terminals of the first and second transistors. As an alternative, the common terminals of the first and second transistors may be connected via respective second resistances to a first current source.
Where the transistors are bipolar transistors, the input, output and common terminals are the base, collector and emitter terminals, respectively. Where the transistors are field effect transistors, the input, output and common terminals are the gate, drain and source terminals, respectively.
The second transconductance stage may comprise third and fourth transistors whose input terminals comprise the second input and whose output terminals comprise the second output. The common terminals of the third and fourth transistors may be connected via respective third resistances to a power supply line. The third and fourth transistors may be of the same conductivity type as the first and second transistors. As an alternative, the common terminals of the third and fourth transistors may be connected via respective fourth resistances to a second current source. The third and fourth transistors may be of a conductivity type opposite that of the first and second transistors.
The third transconductance stage may comprise fifth and sixth transistors whose control terminals comprise the third input and whose output terminals comprise the third output. The common terminals of the fifth and sixth transistors may be connected via respective fifth resistances to a third current source. The input terminals of the first and second transistors may be connected to the input terminals of the fifth and sixth transistors, respectively, and the output terminals of the first and second transistors may be connected to the output terminals of the sixth and fifth transistors, respectively.
According to a second aspect of the invention, there is provided a tuner including an amplifier according to the first aspect of the invention.
It is thus possible to provide an amplifier which is suitable for use at radio frequencies and which provides improved linearity while maintaining a low noise figure. These improvements are obtained by providing a combination of feedforward error correction and feedback so that the distortion performance is improved without compromising the noise figure.